1. Field of the Invention
The present invention relates to a sheet feeding apparatus, and more particularly, it relates to a sheet re-feeding mechanism for re-feeding a sheet from an intermediate tray in an image forming system capable of performing a two-sided printing operation and a multi-print operation.
2. Related Background Art
Conventional apparatus, a two-sided image forming system generally includes an intermediate tray for receiving and stacking sheets each having an image printed on a first surface. After the image forming operation regarding first surfaces of the sheets has been completed, the sheets stacked in the intermediate tray are separated and fed one by one to reinsert each sheet into an image forming portion, where an image is printed on a second surface of the sheet. An important matter in performing such a two-sided image forming operation is the reliability of the sheet feeding, i.e., whether the sheet can be fed properly or not. Particularly, the reliability of the sheet re-feeding is important. Whether the sheet having the image printed on its first surface can be accurately separated and individually fed from the intermediate tray is a problem.
FIG. 11 shows an example of a conventional two-sided image forming system.
In FIG. 11, the image forming system includes a body or frame 1, an original support 2 on which an original is rested, a photosensitive drum 3 which can bear an image from the original, mirrors 4, 5, 6, 7 for directing the image on the original to the photosensitive drum 3, a focusing lens 8 for focusing the image on the photosensitive drum, and a transfer electrode 9 for transferring a toner image formed on the photosensitive drum onto a sheet P.
The sheet P stored in a cassette 10 is picked up by a supply roller 11 and is fed out by regist rollers 12 at a predetermined timing synchronous with the image formed on the photosensitive drum 3. After the toner image has been transferred onto the sheet by the transfer electrode 9, the toner image is fixed to the sheet by means of a fixing device 13. When an image is to be superimposed on the first surface of the same sheet P, the sheet P is directed to a sheet feeding path 15 by a switching guide 14 and is then stored in an intermediate tray 16.
On the other hand, when an image is to be formed on a second surface, opposed to the first surface, of the same sheet, the sheet P is directed to a sheet feeding path 17 by the switching guide 14 and is ejected halfway by means of ejector rollers 18. Then, by reversing the ejector rollers 18, a trailing end of the sheet P is introduced into a sheet feeding path 20 by means of a switching guide 19 and then is stored in the intermediate tray 16.
Thereafter, when the image is printed again on the first or second surface of the sheet, the sheet P having the image formed on its first surface is re-fed from the intermediate tray 16 individually and is sent to the image forming portion, where the image is printed on the second surface of the sheet. Now, since the sheet P having the first surface on which the image was formed has been passed through the fixing device 13 to fix the image onto the first surface, the sheets P stacked in the intermediate tray 16 may include the silicone oil adhered on its imaged surface (first surface) or may have a curled end portion formed due to heat and pressure in the fixing device. Thus, there arises a problem that the double feed and/or the jamming of the sheet will occur in the re-feeding of the sheet from the intermediate tray.
FIG. 12 shows an example of a two-sided image forming system designed to enhance such reliability. In this conventional image forming system, whenever the sheet is introduced into the intermediate tray 16, the whole sheet stack is shifted slightly by means of a pair of conveying rollers 21. so that the sheets are stacked in a stepped fashion.
The sheet stack overlapped in such stepped fashion is wholly conveyed toward a pair of rollers 22 after the image forming operation regarding first surfaces of the sheets has been completed. Then, by stopping the activation of the paired conveying rollers 21 immediately after a lowermost sheet P in the sheet stack leaves the paired conveying rollers 21, only the lowermost sheet P pinched between the paired rollers 22 is fed out by such rollers 22 for the second surface printing operation. According to this example, it is possible to improve the prevention of the double-feed of the sheets during the sheet refeeding operation.
Incidentally, in performing the two-sided printing operation, the technique that the sheets each having the image printed on its first surface are stacked in the stepped fashion is disclosed in the Japanese Patent Laid-open No. 58-178373, Japanese Patent Publication No. 63-18744 and the like.
However, in the above conventional example, it is necessary to provide or prepare a condition that only the lowermost sheet P in the sheet stack is pinched by the paired rollers 22 and at the same time is separated from the paired rollers 21, and the remaining sheets are pinched by only the paired rollers 21. To do so, as shown in FIG. 13A, it is necessary to set a distance between the paired rollers 21 and 22 to a length slightly shorter than a length of the sheet P. In this case, although the toner fixed to the sheets does not rub with each other and thus the sheets are not smeared, the sheets P having different sizes cannot be re-fed.
Accordingly, if it is desired to re-feed the sheets P having different sizes, the optimum positional relation between the paired rollers 21 and 22 as shown in FIG. 13A must be satisfied regarding the smallest size sheet P and at the same time a condition as shown in FIG. 13B must be satisfied regarding the larger size sheets. That is to say, only the lowermost sheet to be re-fed is separated from the paired rollers 21 and the remaining sheets are pinched by the paired rollers 21, and at least the lowermost sheet is pinched by the paired rollers 22. Thus, in this case, when the sheets P having the sizes sufficiently larger than the smallest sheet size are fed, two or more sheets including the lowermost sheet are pinched by the paired rollers 22. From this condition, a rubber driving roller 22b of the paired rollers 22 shifts the lowermost sheet. The remaining sheets are not shifted since they are pinched by the paired rollers 21.
Consequently, as shown in FIGS. 14 and 15, the lowermost sheet P.sub.1 is shifted toward a direction shown by the arrow while slidingly contacting with a second or upper sheet P.sub.2. Thus, during the multi-print operation, as shown in FIG. 14, the toner particles on the second sheet P.sub.2 in the nip area (between the rollers 22) are transferred onto a hatched area of the lowermost sheet P.sub.1 and thus are peeled from the second sheet P.sub.2. On the other hand, during the two-sided printing operation, as shown in FIG. 15, the toner particles on the lowermost sheet are transferred onto the second sheet P.sub.2 at the nip area, and thus, the toner particles on the hatched area of the lowermost sheet P.sub.1 are peeled from the lowermost sheet.